This invention relates to a method for reducing fouling and sliming of submerged objects or marine structures for extended periods of time.
The fouling of boat hulls and other objects immersed in sea water is a well-known problem. Fouling is a term used to describe the damage caused by the growth of certain marine macro-organisms such as barnacles, mollusks, incrusting bryozoans, annelids, hydroides, algae, etc. It is well-known that these organisms are highly susceptible to copper. Accordingly, for centuries sailing vessels have been clad with copper hulls. More recently copper has been applied to hulls in the form of paints, polymers, or copper containing polymers.
Perhaps the most common method of preventing the fouling of a boat hull, particularly on small crafts, is the use of antifouling paints. Such paints are applied to all surfaces of the hull exposed to water for any prolonged period of time. Indeed, even fiberglass hulls, which otherwise do not require painting, are normally painted at the beginning of each boating season with an anti-fouling paint on areas of the hull which will become exposed to water for long periods of time during the boating season. It would, of course, be highly desirable to provide a gel coat for fiberglass hulls which is anti-fouling for several years.
In addition to the prevention of fouling, the present invention can afford resistance to sliming. Sliming is a term used to describe the opaque film which forms on the surface of submerged objects. Sliming is generally caused by microorganisms such as bacteria. Indeed, when a vessel is placed in salt water, initially bacteria becomes associated with the surface of the object to form a film of slime. The association of the bacteria is followed by an association of a biotic progression of diatoms, hybrides, algae, bryozoans, protozoans, and finally macro-organisms (foulents). It is desirable to reduce sliming for many reasons. Obviously, the presence of slime contributes to problems associated with foulents such as increasing drag, increasing fuel consumption, etc. However, perhaps more significant is the fact that macro-organisms tend to be rugophilic and settle on roughened surfaces in preference to smooth surfaces. Thus, there is a view that marine slimes precondition the surface of submerged objects in a manner that stimulates the settling of foulents. Thus, slime provides the physical substrate and possibly a nutriant source which encourages the attachment of macroscopic plants and animals. Obviously, it would be desirable to provide a material which not only prevents fouling but which also reduces sliming. This fact is significant because antifouling paints are not generally characterized as being anti-sliming. The conventional method of preventing sliming is to chemically treat the water, i.e., chlorination or ozonation.
Of course, fouling and sliming is also a problem which occurs when objects are in contact with fresh water. Thus, the present invention is also directed to the coating of objects which remain in contact with fresh water for prolonged periods of time.
As has been stated above, a common procedure for preventing a boat hull from fouling is to apply an anti-fouling paint. Typically, an anti-fouling paint contains an anti-fouling metal such as copper which leaches into the water to form copper ions. These ions are deadly to the foulents. As one particle of the anti-fouling metal dissolves, another particle is exposed to solvolysis. This leaching is rapid (when compared to the leaching rate of solid copper) and varies with such factors as coating age, temperature of the water, salinity of the water, etc.
In order to fully use the toxicant in an anti-fouling resin, conventional paints depend on the solubility or wearing away of the matrix. This is necessary since most matrices entirely encapsulate the toxic component and, if not soluble, would prevent it from being effective. This mechanism is inefficient since the toxicant is often released into the water before it is entirely consumed.
It has been discovered and disclosed in U.S. Pat. No. 4,197,233 that the antifouling properties of a copper-containing polymeric coating can be enhanced by incorporating the copper in flake form.